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6, 21, 155, 889, 106483, 2228207, 9961453, 66571993057, 140656423562035331011, 55088331748199422233011027879, 17361742620725829882898847100829589, 21607930299479592429924287571917281427329
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OFFSET
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1,1
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COMMENTS
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Note that a(3)=155 and a(4)=889 also belong to A119691.
Old name was: Composite numbers such that the first factor is the relevant exponent of Mersenne prime (the second factor).
a(n) is the smallest k > 0 such that A000668(n)^2 divides 2^k-1. Then lpf(2^k-1) = A000668(n), where lpf(m) = A020639(m). - Thomas Ordowski, Feb 03 2019
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LINKS
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Table of n, a(n) for n=1..12.
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FORMULA
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a(n) = A000043(n)*A000668(n). - R. J. Mathar, Oct 18 2007
a(n) = A002326((A000668(n)^2 - 1)/2). - Thomas Ordowski, Feb 03 2019
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PROG
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(PARI) expm = [2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127]; vector(#expm, i, expm[i]*(2^expm[i]-1)) \\ where expm comes from A000043; Michel Marcus, Feb 07 2019
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CROSSREFS
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Cf. A000043, A000668, A002326, A020639, A119691.
Sequence in context: A304264 A210443 A179768 * A244299 A143049 A213680
Adjacent sequences: A131957 A131958 A131959 * A131961 A131962 A131963
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KEYWORD
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nonn,less
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AUTHOR
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A.K. Devaraj, Aug 02 2007, Aug 06 2007
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EXTENSIONS
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Terms corrected, edited, and new name by Michel Marcus, Apr 30 2013
a(12) from Michel Marcus, Feb 07 2019
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STATUS
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approved
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